This invention relates to a method of ensuring the maintenance of constant quality of spot welds.
In spot welding a low voltage current of high amperage is passed between two electrodes through the lapped joint of two components that are to be welded, and a local weld is formed by the resultant generation of heat and the application of pressure. The total welding resistance is composed of the resistances of the materials forming the joint and of contact resistances in the path of the current. In the initial stage the contribution made by the contact resistances generally far outweighs that of the material of the components. However, as welding proceeds the contact resistances become rapidly lower. Their rate of decrease and the minimum they reach depend materially upon the condition of the surface of the welded components, upon the magnitude of the current, the pressure exerted by the electrodes as well as the electrode geometry. Furthermore, the change in resistance at the points of contact between electrode and metal will be affected by differences in specific pressure and in current density. If the state of the surface of the two components varies, the generation of heat in the weld nugget will also vary and the distribution of heat will be irregular although the settings of the welding machine remain unchanged. The result is the production of welds of irregular quality.
There is a particular demand for some method of automatically ensuring the production of constant quality welds for the spot welding of panels in the sheet metal industry. Various equipment is already known for improving the reproducibility of standard quality welds. Generally speaking, this can be classified under four different groups:
(a) Measuring instruments for measuring and indicating one or more of the parameters that are essential to the welding process, e.g. such as the r.m.s. value of the welding current.
(b) Control Instruments which assess the quality of a finished weld by measuring and comparing a prescribed quantity which is specific to the process with a reference value and which indicate the upward or downward deviation when this exceeds a prescribed limit of tolerance. The welding machine can be switched off if the deviation is abnormal.
(c) Quality Control Instruments which function in the same way as the instruments under (b), but which also trigger action for the readjustment of particular welding parameters before the next weld is made.
(d) Fully Automatic Closed Loop Control Systems which continuously conduct and correct the process by a continuous comparison between measured and reference quantities.
The instruments included in the above groups (c) and (d) are functionally based on one of the following principles of operation:
1.1 The maintenance of a constant voltage and/or current on the primary side of the welding transformer.
1.2 The maintenance of a constant voltage and/or current on the secondary side of the welding transformer.
1.3 A voltage integration over the welding time with time and current limitation.
1.4 A current integration over the welding time with time and current limitation.
1.5 Power integration as the product of the transient values of voltage and current during the welding time with current limitation (constant power system).
1.6 The control of welding time by reference to the thermal expansion or the rate of thermal expansion of the components at the weld nugget in a direction normal to the metal surface.
1.7 Control of welding time by reference to overall resistance.
However, none of these instruments is capable of ensuring the reproducibility of the welds under all conditions that may in practice arise.
For instance, instruments which measure electrical quantities cannot determine how much of the current is shunted through a neighboring weld that has already been completed or through a burr in the panel. Arrangements which rely on the displacement of the electrodes by thermal expansion of the components at the weld for purposes of control usually make no allowance for tolerances in the gauge of the sheet material and they can only partly compensate for electrode wear and fluctuations in other welding parameters.
In welding machines which are thus controlled extraneous interference with the welding process and gradual changes in the welding parameters in the course of a number of welds may also inadmissibly adversely affect the welding results, either in a random way in the case of a single weld, or gradually after a sequence of good spot welds have been made, although satisfactory results should have been expected from the preset reference parameters in a fully automatically controlled machine.
In the sheet metal processing industries faulty welds continue to occur although the starting conditions are correct; non-destructive testing often and at least sometimes completely fails to detect them. Manual spot checks--for instance with a chisel--are very unreliable and not satisfactory. Moreover, they require work to cease, when a given number of spot welds have been made, to enable an electrode or electrode cap to be replaced or remachined to exclude effects which are due to changes in the nature of the electrode contact face and the electrical contacting conditions, and which cannot be compensated by the above mentioned methods of control.
There is as yet no system which permits varying starting conditions, particularly respecting the initial electrical resistance, to be systematically controlled by the machine at the beginning of each welding operation.